No 3 (2024)

For the first time, authigenic dispersed biotite was discovered in Pleistocene terrigenous sediments of the Central Hill, located in the Escanaba Trough in the southern part of the Gorda Ridge (northeastern sector of the Pacific Ocean), which accounts for almost the entire content of fine fractions <1 μm of some samples from ODP 1038B hole. The authigenic nature of biotite is associated with the metasomatic effect of hydrothermal solution on terrigenous clay minerals after intrusion of basaltic magma in the form of laccolith with a temperature of ~1200°C. The mineral composition of fine fractions of sediments was studied using complex analytical methods, including modeling of their diffraction patterns. It has been established that the dispersed micaceous mineral (biotite) is trioctahedral, high-iron, does not swell when saturated with glycol, but contracts after heating at 550°C. It is shown that in its structure there are no foreign layers, the height and composition of which differ from the micaceous layers. The decrease in the height of micaceous layers upon heating biotite to 550°C is mainly due to a decrease in the height of 2:1 octahedra due to the difference in the Fe²⁺–O and Fe³⁺–O bond lengths as a result of the oxidation of Fe²⁺ cations It has been established a limit value for the coefficient of variation CV, characterizing the absence of mixed-layering in a regular structure, which should not exceed a value of 0.10.

A technique that allows to evaluate the degree of roundness of detrital zircon grains on a five-point scale according to the degree of preservation of the vertices, edges and faces of the crystals is presented. Based on the results of the degree of roundness of individual grains, the integral index SOs is calculated for a sample from various stratigraphic units. The obtained data makes it possible to determine the proportion of grains of different classes in different strata, to assess the degree of similarity/difference of samples, which can be an additional criterion when dividing sedimentary sections. The proposed method is used to compare the roundness characteristics of detrital zircons from a number of sedimentary sequences of the Cimmerides of the Crimean Mountains. It is shown that the sandstones of the Chenka sequence, relative to the sandstones of the flysch sequences, are characterized by a higher proportion of unrounded and weakly rounded zircon grains and a virtual absence of completely rounded grains. The values of the integral roundness index SOs for zircons from sandstones of the Tauride Groupe have showed values of 3.41 and 3.95, and for the Chenka sequence – 2.55. Thus, a significant difference in SOs was recorded between the studied strata, which is also confirmed by the values of the mutual pair coefficients p, calculated using the Kolmogorov–Smirnov test.

The paper presents new data from a comparative analysis of sedimentary rocks from the Mendeleev Rise (seamount Shamshura), presumably of Triassic age, with sandstones of Aptian age, obtained during underwater sampling of the Alpha-Mendeleev Rise during expeditions in 2012, 2014 and 2016. The geochemical characteristics of the rocks are very similar, and, as a rule, data from samples of different ages form common fields on different diagrams. The petrographic investigations have established that the sandstones of presumably Late Triassic age, as well as the sandstones of Chukotka and Wrangel Island, are dominated by fragments of shale and acid effusives. The Aptian samples are dominated by fragments of mafic rocks. Also, sharp differences are observed in the nature of the age spectra of detrital zircon populations, indicating fundamentally different feeding provinces for the Triassic and Cretaceous sandstones of the Alpha-Mendeleev Rise. Samples from the Shamshura seamount are characterized by populations similar to populations from Triassic rocks of Chukotka and Wrangel Island, indicating the presence of Triassic rocks in this part of the Mendeleev Rise.